HVAC Chillers

HVAC Chillers — Practical Design & O&M Guide

1) What a chiller does (in one line)

Removes heat from a building’s chilled-water loop via a vapor-compression (or absorption) cycle and rejects it to air (air-cooled) or water (water-cooled/tower).

2) Main classifications

• By heat rejection
  • Air-cooled: Outdoor condenser coils + fans; quick install; higher kW/ton; great where water is scarce.
  • Water-cooled: Condenser water loop + cooling tower; higher efficiency; more infra/maintenance.
  • Evaporative-cooled: Hybrid (less common).
• By compressor
  • Centrifugal (large tonnage, high efficiency, oil or magnetic bearings).
  • Screw/rotary (mid-to-large, solid part-load).
  • Scroll (small-to-mid, modular).
  • Reciprocating (legacy/small).
• By drive/source
  • Electric (most common), steam/absorption (waste heat/CHP integration).
• Special
  • Heat-recovery chillers (simultaneous cooling + hot water).
  • Turbocor/magnetic-bearing (ultra-low part-load power, low maintenance).

3) Core components (what to look for)

• Compressor (type, bearings, VFD).
• Evaporator (shell-and-tube or brazed plate; insulation, freeze protection).
• Condenser (tube materials, water quality).
• Expansion device (EEV vs TXV).
• Controls (BMS integration, safeties, trending).
• Accessories: oil management, purge unit (centrifugal), vibration isolation.

4) Performance metrics & quick math

• Cooling capacity:
  Tons=500×GPM×ΔTCHW12,000\text{Tons} = \dfrac{500 \times \text{GPM} \times \Delta T_{CHW}}{12{,}000}Tons=12,000500×GPM×ΔTCHW​​
• Flow thumb rules:
  • Chilled water: GPM≈24×TonsΔTCHW(°F)\text{GPM} \approx 24 \times \dfrac{\text{Tons}}{\Delta T_{CHW}(°F)}GPM≈24×ΔTCHW​(°F)Tons​
    (e.g., 12°F ΔT ⇒ ~2 GPM/ton; 10°F ⇒ ~2.4 GPM/ton)
  • Condenser water: 2.5–3.0 GPM/ton (range 9–12°F typical)
• Efficiency:
  • kW/ton (lower = better) and COP = 3.517 / (kW/ton).
  • IPLV/NPLV for part-load (plant rarely at 100%—optimize for 30–70%).
• Approach temps:
  • Evap approach (CHWS – evap sat), Cond approach (cond sat – CWS out).
  • Tighter approach = better HX/tube cleanliness.

5) Refrigerants (selection notes)

• Favor low-GWP options (e.g., R-1234ze, R-514A, newer A2L blends) where code-permitted.
• Ensure ASHRAE 34 safety classification, charge limits, ventilation, leak detection, and local code compliance.

6) Plant configurations (controls matter more than bronze)

• Primary–secondary: Constant chiller flow; variable secondary loop. Stable, easy to retrofit.
• Variable primary flow (VPF): Fewer pumps, better part-load. Needs min-flow protection/bypass logic per chiller.
• Series counterflow chillers: Excellent lift reduction at part-load in high ΔT systems.
• Heat-recovery: Use condenser heat for DHW/reheat (shoulder seasons = big wins).
• Tower control (water-cooled):
  • Condenser water setpoint reset toward ambient wet-bulb (respect min LMTD/cond approach).
  • Fans VFD; cell staging; freeze protection.

7) Water-side design (numbers you use every day)

• CHW ΔT: 12–16°F modern designs (legacy 10°F). Higher ΔT = smaller pipes/pumps, better plant IPLV.
• Design velocities:
  • CHW mains: ~6–10 ft/s (balance noise/head).
  • Condenser water: ~6–8 ft/s (mitigate fouling, erosion).
  • Evaporator/Condenser tubes: 3–12 ft/s (per OEM water quality).
• Pump heads: Sum coil/valve/HX + risers + fittings + safety margin; keep NPSH available > NPSH required.
• Water quality: Filtration, side-stream filters, chemical treatment, biocide, tube material compatibility (CuNi/SS).

8) Electrical & controls integration

• VFD on compressors (screw/centrifugal) + VFD on pumps and tower fans.
• BMS points: starts/stops, safeties, kW, entering/leaving temps, ΔP, valve positions, alarms, trend logs.
• Sequencing: Start smallest/highest-IPLV first, load to sweet spot (e.g., 0.5–0.7 kW/ton), then enable next.
• Setpoint strategies:
  • CHWS reset upward when loads are light and coils still satisfy (e.g., 42→46°F).
  • DP reset on CHW loop from most-open valve feedback.

9) Selection workflow (10 steps)

1. Define load profile (peak vs typical, diversity, growth).
2. Choose air- vs water-cooled (site water/space/noise/efficiency).
3. Pick ΔT targets (CHW 12–16°F; CW range 9–12°F; approach 5–7°F).
4. Estimate flows (GPM) & pump heads.
5. Shortlist compressor tech (efficiency, turndown, acoustics, maintenance).
6. Check ambient/wet-bulb bins and part-load—optimize IPLV/NPLV.
7. Validate refrigerant (safety class, codes).
8. Lay out plant topology (VPF vs P-S, headers, bypass, strainers, expansion tank).
9. Plan controls & sequences (resets, staging, min-flow).
10. Life-cycle cost: CapEx + energy + water + maintenance (10–20 yrs).

10) Commissioning checklist (abridged)

• Piping flushed; strainers clean; air vents/air separator working.
• Sensor calibration (temp, DP, flow).
• Pump curves verified; VFD min/max set; no hunting.
• Chiller safeties & interlocks proven (flow, freeze, surge, oil).
• Tower: cell isolation, basin heaters, level controls, drift eliminators, water treatment online.
• Trend logs enabled (kW/ton, CHWS/L, CWS/L, DP, valve positions).

11) O&M essentials

• Routine: tube cleaning (condenser more frequent), water treatment, oil/refrigerant analysis (as applicable), vibration & bearing checks, purge op (centrifugal), strainer cleaning.
• Keep ΔT high: fix 3-way valves, balance coils, reset DP.
• Typical faults & quick pointers:
  • Low ΔT (“low ΔT syndrome”): oversized 3-way coils, bypassing, poor balancing ⇒ convert to 2-way, tune valves, DP reset.
  • High kW/ton: dirty tubes, high lift (too warm CW), non-condensables, wrong CHWS setpoint.
  • Nuisance trips: unstable flow (VPF min-flow), sensor drift, air in system.

12) Mini case study (sample sizing)

Goal: 1,200-ton water-cooled plant, CHW ΔT = 12°F, CW range = 10°F.

• CHW flow: ≈24×1200/12≈2,400\approx 24 \times 1200 / 12 \approx 2{,}400≈24×1200/12≈2,400 GPM.
• CW flow: ~3 GPM/ton ⇒ ~3,600 GPM.
• Tower: Size for ~1,200 tons at design WB; approach 5–7°F; VFD fans + multi-cell.
• Topology: 3 × 400-ton centrifugal/screw with VFD, VPF piping, DP reset, CHWS reset 42→46°F.
• Expected: <0.55–0.65 kW/ton at typical loads if tubes clean and resets active.

13) Templates you can reuse

A) Submittal review essentials

• Rated tons @ ARI conditions, IPLV/NPLV, sound power, min leaving CHWS, min/max flows, min-flow protection, approach temps, allowable glycol %, short-circuiting tolerance, E-stop & safeties, BACnet points list.

B) Controls sequence (snip)

• Enable chiller when: CHW loop ΔP < setpoint OR warmest zone valve > 90% for 5 min.
• Stage logic: Load first unit to 65–75% before starting next; unload last on with falling load.
• Resets:
  • CHWS = 42–46°F based on warmest SAT/valve position.
  • CW setpoint = WB + approach, bounded by min LMTD & OEM min cond water.

C) Maintenance schedule (high level)

• Weekly: trends review (kW/ton, ΔT), strainers, visual leaks.
• Monthly: water chemistry, tower drift & fill inspection, vibration trend.
• Quarterly: tube delta-P review, oil/refrig sample (per OEM), test safeties.
• Annual/off-season: tube cleaning, eddy-current test (as needed), calibration.